The invention relates to a method for the catalyzed reparation of methyl and ethyl esters of trifluoroacetic and chlorodifluoroacetic acid.
The methyl and/or ethyl esters of trifluoroacetic and/or chlorodifluoroacetic acid may be used as solvents or cleaning agents. For example, ethyl trifluoroacetate is a solvent in the chlorination of paraffin. The esters are also intermediate products in chemical synthesis. Methyl trifluoroacetate and 1,1,1-trifluoroethyl trifluoroacetate yield trifluoroethanol (and possibly methanol) after hydrogenation. Trifluoroethanol is a solvent and also an intermediate product for the preparation of isofluorane, an anaesthetic. The methyl and ethyl esters of chlorodifluoroacetic acid are likewise synthetic building blocks, for example for the production of liquid crystals.
U.S. Pat. No. 5,405,991 (=EP-A-0,623,582) discloses the preparation of esters of trifluoroacetic acid and of chlorodifluoroacetic acid from the acid chlorides and the corresponding alcohol in the presence of alkali metal or xe2x80x9coniumxe2x80x9d salts of the carboxylic acid corresponding to the carboxylic acid chloride used.
It is the object of the present invention to obtain substantially pure esters (degree of purity  greater than 94% by weight) in an even simpler manner.
This and other objects have been achieved in accordance with the present invention by providing a method for preparing a methyl or ethyl ester of trifluoroacetic acid or chlorodifluoroacetic acid, comprising reacting an acid chloride of trifluoroacetic acid or chlorodifluoroacetic acid with a stoichiometric excess of methyl or ethyl alcohol in the presence of an onium salt of the acid as a catalyst, the molar ratio of alcohol to acid chloride being selected such that two phases are formed, one phase being an ester phase which without a distillation step contains the ester in a purity of at least 95% by weight.
The invention is based on the observation that in the preparation of the methyl or ethyl esters of trifluoro-acetic acid or of chlorodifluoroacetic acid from the acid chlorides and the alcohol in the presence of xe2x80x9coniumxe2x80x9d salts of the relevant acids as catalyst, at certain molar ratios two phases form, namely such that in the two-phase region one phase always comprises the substantially pure ester.
The method according to the invention for the preparation of methyl or ethyl esters of trifluoroacetic acid and of chlorodifluoroacetic acid provides for the acid chloride to be reacted with an excess of the alcohol in the presence of an onium salt of the acid in question and for the molar ratio of alcohol to acid chloride to be selected such that two phases are formed, one phase containing the ester in a purity, achievable without a distillation stage, of at least 95% by weight. The ester can be isolated by separating the ester phase from the other phase. Using this procedure, the ester is thus produced in a purity which makes distillation unnecessary. One preferred embodiment of the method according to the invention therefore provides for the isolation of the resulting ester without distillation. The molar ratio of alcohol to acid chloride is advantageously in the range from 1.01:1 to 5:1.
When preparing the methyl ester of trifluoroacetic acid, the molar ratio of methanol to trifluoroacetyl chloride is in the range from 1.03:1 to 4:1. When preparing the ethyl ester of trifluoroacetic acid, the molar ratio of ethanol to trifluoroacetyl chloride is in the range from 1.01:1 to 5:1. When preparing the methyl ester of chlorodifluoroacetic acid, the molar ratio of methanol to chlorodifluoroacetyl chloride is in the range from 1.06:1 to 2.5:1. When preparing the ethyl ester of chlorodifluoroacetic acid, the molar ratio of ethanol to chlorodifluoroacetyl chloride is in the range from 1.02:1 to 2.5:1. In the above ranges, there are two phases in which, as stated, one phase comprises the ester, which is always contained in a purity of at least 95% by weight. The methyl esters always form the lower phase; the ethyl ester of chlorodifluoroacetic acid likewise forms the lower phase, and the ethyl ester of trifluoroacetic acid forms the upper phase.
As in the generic method, addition of an acid is not necessary, and is preferably not effected. Preferably the method is performed continuously.
As used herein, the term xe2x80x9coniumxe2x80x9d refers to cations having positively charged nitrogen, for example protonated aromatic nitrogen bases such as pyridinium or protonated alkyl-, dialkyl- or trialkylammonium cations having up to 20 carbon atoms, or for ammonium compounds substituted by cycloalkyl, or cycloaliphatic nitrogen bases such as piperidinium, or quaternary ammonium cations.
Highly suitable carboxylic acid salts. are xe2x80x9coniumxe2x80x9dsalts, xe2x80x9coniumxe2x80x9d standing for a cation of nitrogen of the formula Rxe2x80x2Rxe2x80x3Rxe2x80x2xe2x80x3Rxe2x80x3xe2x80x3N+, in which Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 and Rxe2x80x3xe2x80x3. independently of each other, stand for hydrogen, alkyl with 1 to 20 carbon atoms, aryl or aralkyl, or wherein Rxe2x80x2 and Rxe2x80x3 or wherein Rxe2x80x2xe2x80x3 and Rxe2x80x3xe2x80x3, or wherein Rxe2x80x2, Rxe2x80x3 and Rxe2x80x2xe2x80x3, or wherein Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 and Rxe2x80x3xe2x80x3, optionally with inclusion of the nitrogen atom, form saturated or unsaturated ring systems. xe2x80x9cArylxe2x80x9d here means in particular phenyl, or phenyl substituted by one or more C1-C2-alkyl groups. Salts in which xe2x80x9coniumxe2x80x9d stands for ammonium, pyridinium or R1xe2x80x2R2xe2x80x2R3xe2x80x2R4xe2x80x2N+, in which R1xe2x80x2, R2xe2x80x2, R3xe2x80x2 and R4xe2x80x2, independently of each other, are hydrogen, alkyl with 1 to 15 carbon atoms, phenyl or benzyl, are especially suitable. Examples of such cations include pyridinium, piperidinium, anilinium, benzyltriethylammonium and triethylammonium.
The temperature at which the reaction is performed is best from ambient temperature (about 20xc2x0 C.) up to the boiling point of the mixture, for example up to 100xc2x0 C. Operation is at ambient pressure (approximately 1 bar absolute) or if desired also at elevated pressure, for example at a pressure of up to 5 bar absolute.
The xe2x80x9coniumxe2x80x9d salt may be present in catalytic or molar quantities. Advantageously, the molar ratio of the acid halide to the carboxylic acid salt (onium salt) lies within the range from 1:1 to 20:1, but it is possible also to operate outside this range. Surprisingly, the presence of the onium salt results in a greater purity of the ester phase. The optimum quantity of onium salt for achieving a desired purity can be readily determined by orienting tests, simply by analyzing the ester phase in each case, e.g. by a gas chromatogram.
According to one particular embodiment of the invention, the acid chloride and the xe2x80x9coniumxe2x80x9d salt of the carboxylic acid are produced in situ. To this end, the corresponding xe2x80x9coniumxe2x80x9d chloride is reacted with the anhydride of the carboxylic acid to be used. During this reaction, the corresponding acid halide and the corresponding salt are formed from the anhydride of the carboxylic acid. With this embodiment, spent halide catalysts can be used as alkali metal or xe2x80x9coniumxe2x80x9d halide, and in this manner can be converted into valuable products. Preferably pyridinium salts are used.
The invention has the advantage that it is possible to produce carboxylic acid esters without hydrolytic working-up in a technically particularly simple, energy-saving manner.
The invention will be explained in greater detail with reference to the following examples, without being restricted in its scope.
Preparation of Trifluoroacetic Acid Alkyl Esters